syntax: Remove #[non_exhaustive] from Edition

`Edition` is not a public API, we want users to break when a new edition is added

src/librustc/ich/impls_syntax.rs

@@ -134,14 +134,10 @@ impl_stable_hash_for!(struct ::syntax::attr::Stability {
     const_stability
 });
 
-impl<'a> HashStable<StableHashingContext<'a>>
-for ::syntax::edition::Edition {
-    fn hash_stable<W: StableHasherResult>(&self,
-                                          hcx: &mut StableHashingContext<'a>,
-                                          hasher: &mut StableHasher<W>) {
-        mem::discriminant(self).hash_stable(hcx, hasher);
-    }
-}
+impl_stable_hash_for!(enum ::syntax::edition::Edition {
+    Edition2015,
+    Edition2018,
+});
 
 impl<'a> HashStable<StableHashingContext<'a>>
 for ::syntax::attr::StabilityLevel {

src/librustc/ty/context.rs

@@ -1493,12 +1493,6 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             BorrowckMode::Ast => match self.sess.edition() {
                 Edition::Edition2015 => BorrowckMode::Ast,
                 Edition::Edition2018 => BorrowckMode::Migrate,
-
-                // For now, future editions mean Migrate. (But it
-                // would make a lot of sense for it to be changed to
-                // `BorrowckMode::Mir`, depending on how we plan to
-                // time the forcing of full migration to NLL.)
-                _ => BorrowckMode::Migrate,
             },
         }
     }

src/libsyntax/ext/tt/quoted.rs

@@ -444,7 +444,6 @@ where
             macro_node_id,
         ),
         Edition::Edition2018 => parse_sep_and_kleene_op_2018(input, span, sess, features, attrs),
-        _ => unimplemented!(),
     }
 }
 

src/libsyntax_pos/edition.rs

@@ -13,7 +13,6 @@ use std::str::FromStr;
 
 /// The edition of the compiler (RFC 2052)
 #[derive(Clone, Copy, Hash, PartialEq, PartialOrd, Debug, RustcEncodable, RustcDecodable, Eq)]
-#[non_exhaustive]
 pub enum Edition {
     // editions must be kept in order, oldest to newest
 

src/libsyntax_pos/hygiene.rs

@@ -17,7 +17,7 @@
 
 use GLOBALS;
 use Span;
-use edition::Edition;
+use edition::{Edition, DEFAULT_EDITION};
 use symbol::Symbol;
 
 use serialize::{Encodable, Decodable, Encoder, Decoder};
@@ -217,7 +217,7 @@ impl HygieneData {
                 opaque_and_semitransparent: SyntaxContext(0),
             }],
             markings: FxHashMap::default(),
-            default_edition: Edition::Edition2015,
+            default_edition: DEFAULT_EDITION,
         }
     }